1. Field of the Invention
This invention relates generally to a corona ignition system and method for igniting a mixture of fuel and air of a combustion chamber.
2. Description of the Prior Art
Corona ignition systems are often preferred for providing robust ignition without the high temperatures and related consequences of conventional spark ignition systems. The corona ignition system includes an igniter having an electrode extending into a combustion chamber. The ground is provided by walls of the combustion chamber or a piston reciprocating in the combustion chamber. The igniter does not include a ground electrode. The electrode of the igniter receives energy from an energy supply and emits an electrical discharge, preferably in the form of a corona discharge. A corona discharge is an electrical field including a plurality of ionized streamers having high electrical impedance from the electrode to the ground. When fuel is supplied to the combustion chamber, the electrical field ignites the mixture of fuel and air in the combustion chamber. An example of a corona ignition system is disclosed in U.S. Pat. No. 6,883,507 to Freen.
As energy is supplied the electrode, the concentration of ions in the electrical field increases. A high voltage is preferred to provide a robust corona discharge. However, if the voltages increases beyond a certain threshold, the increasing ion concentration results in a cascading process typically causing the corona discharge to transform into an arc discharge. An arc discharge is an electrical field including a single streamer providing a conductive path from the electrode to the ground. In typically corona ignition systems, when arc discharge occurs, all of the stored energy of the system is immediately discharged and depleted. The arc discharge may be of short duration and thus not capable of providing reliable ignition. Accordingly, the energy level provided to the electrode is typically at the highest voltage that can provide a corona discharge without switching to an arc discharge.
Oftentimes the voltage passes the corona discharge threshold and the arc discharge occurs. In addition, other situations or engine conditions can cause arc discharge. The arc discharge may also occur when the igniter is fouled by fuel of carbon deposits, or when the piston is too close to the igniter, or during other situations where there is low electrical resistance between the electrode and ground. The arc discharge is typically unintentionally formed and undesirable, but there are certain situations where arc discharge is intentionally formed. In attempt to stop the arc discharge and restore corona discharge, when arc discharge is undesirable, the voltage supplied to the electrode is immediately decreased. However, reducing the voltage is oftentimes not practical or not effective in returning to corona discharge and providing reliable ignition.